The polyamine/epoxy resin systems known from the state of the art are distinguished for example by excellent metal adhesion, very good chemical resistance and outstanding anticorrosive properties. In the case of solvent-containing formulations and powder coating systems, crosslinked films of-high flexibility can be obtained by using epoxy resins with high molecular weights and/or polyaminoamides, e.g. based on dimeric fatty acids, as hardeners. Coatings based on solvent-free liquid resins and solvent-free amine hardeners are very brittle because of the low molecular weights of the epoxy resins and the resulting high network density. Tar substitutes such as coumarone resins, for example, are therefore used in solvent-free formulations as plasticizers. Especially when using relatively large amounts of hydrocarbon resins, such coatings tend towards long-term embrittlement as a result of migration of the non-functional constituents.
Epoxy resins can be given a good and permanent elasticity by combination with polyurethanes. Thus, for example, in DE-A 23 38 256, high-molecular polyether-urethaneureas with amino end groups were prepared by reacting prepolymers containing free isocyanate groups with amines in highly dilute solutions and then cured with epoxy resins. The use of the solvents, especially aromatic solvents, necessary for this purpose is a disadvantage in practice from both the technical and the physiological point of view. On the other hand, the viscosity of the solvent-free reaction products, such as those specifically prepared according to DE-A 23 38 256, is too high for practical applications.
DE-A 24 18 041 describes a process for the production of elasticized mouldings and sheet materials wherein epoxy compounds are reacted with amine compounds obtained by the hydrolysis of prepolymeric ketimines or enamines. Chemically resistant thermosetting moulding compounds with good adhesion and improved properties can be prepared by this process. The process described has the disadvantage of high process engineering costs.
DE-A 21 52 606 describes reactive systems based on alkylphenol-blocked polyisocyanates and polyamines that can optionally also be cured in combination with epoxy resins. Once again, these reactive systems are hampered by a few disadvantages relating to application technology. For example, they have a relatively high viscosity and the blocking agent released is of comparatively low molecular weight, so it migrates out of the coating over time and the adhesion of the coating to the substrate is no longer adequate.
To enable a specific reaction of polyisocyanate prepolymers with excess amounts of diamine, it was therefore proposed in many cases to use the polyisocyanates in blocked form, as described e.g. in CA 1-219 986, EP-A 293 110 or EP-A 082 983, where the preferred blocking agents used are phenols or substituted phenols. Following their reaction with the polyamines, the high boiling point of these substances is such that they can only be distilled from the reaction mixture incompletely, if at all. However, residues of the optionally substituted phenols in the mixtures or in the plastic compound lead to the disadvantages already described.
In EP-A 0 457 089, on the other hand, secondary amines preferably of low boiling point are used as blocking agents. Residues of these amines in the reaction mixture after deblocking readily create an odour nuisance. Although, in principle, the secondary amine used in epoxy systems can be incorporated into the system, this reaction proceeds relatively slowly, especially at low temperatures (e.g. room temperature), whereby part of the amines will leave the coating. In one particularly preferred application, the amine blocking agent is distilled from the reaction mixture after deblocking. Although this procedure yields products that do not create an odour nuisance, it is very involved and hence expensive.
U.S. Pat. No. 6,060,574 further discloses reactive compositions that consist of reversibly blocked organic polyisocyanates and at least one polyamine having at least two primary amino groups, and optionally also comprise a compound containing oxirane groups. Hydrocarbon resins with phenolic OH groups are used as blocking agents for the organic polyisocyanates. Polyisocyanates blocked in this way are distinguished by a markedly reduced reactivity towards polyamines, compared with alkylphenol-blocked polyisocyanates. The organic polyisocyanates used can be prepolymers obtained by reacting polyhydroxyl compounds with an excess of diisocyanates or polyisocyanates. Examples of polyhydroxyl compounds that can be used are polyetherpolyols obtainable by the alkoxylation of suitable starter molecules (e.g. monomeric polyols).
However, all the reversibly blocked polyurethane prepolymers described according to the state of the art and prepared by reacting a polyurethane prepolymer containing isocyanate groups with a blocking agent have the disadvantage that, after the reaction with a polyamine, the blocking agent is released again. The blocking agent is not chemically bonded in the plastic formed, so it can escape or be washed out over time, which is a great disadvantage in terms of the mechanical properties of the plastic.
Furthermore, the reversibly blocked polyurethane prepolymers known hitherto have a high viscosity due to intermolecular hydrogen bridging of the urethane groups, which is a great disadvantage for the processing of corresponding reactive systems with polyamines and optionally epoxides. Because of their high viscosity, such systems cannot usually be applied by spraying.
The object of the present invention was therefore to provide novel polyurethane prepolymers which have a markedly lower viscosity than the reversibly blocked polyurethane prepolymers known hitherto, and which, together with polyamines and optionally compounds containing oxirane groups, can be used in reactive systems that cure at room temperature, the blocking agent not being released when the reactive system cures (elimination-free systems).